To improve thermal efficiency of gasoline internal combustion engines, dilute combustion—using either air or re-circulated exhaust gas—enhances thermal efficiency and reduces NOx emissions. However, there is a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn in one or more cylinders. Known methods to extend the dilution limit include 1) improving ignitability of the mixture by enhancing ignition and fuel preparation, 2) increasing the flame speed by introducing charge motion and turbulence, and 3) operating the engine using a controlled auto-ignition combustion process.
The controlled auto-ignition process is also referred to as Homogeneous Charge Compression Ignition (‘HCCI’) process. In this process, a mixture of combusted gases, air, and fuel, referred to as a combustion charge, is created and auto-ignition is initiated simultaneously from many ignition sites within the mixture during compression, resulting in stable power output and high thermal efficiency. Since the combustion is highly diluted and uniformly distributed throughout the combustion charge, the burnt gas temperature and hence NOx emissions are substantially lower than NOx emissions of a traditional spark ignition engine, based on propagating flame front, and of a traditional diesel engine, based on an attached diffusion flame. Combustion phasing is an important aspect of the combustion process, and comprises timing of an in-cylinder combustion parameter relative to piston position and is typically measured by crankshaft rotational angle. In-cylinder combustion parameters comprise such parameters as location of peak pressure (LPP), and, an engine crank angle at which 50% of a combustion charge is burned (CA-50).
Engines operating under controlled auto-ignition combustion depend on factors including cylinder charge composition, temperature, and pressure at the intake valve closing to control combustion phasing. Hence, the control inputs to the engine, e.g., fuel injection mass and timing (relative to piston position) and intake/exhaust valve profiles, must be carefully coordinated to ensure robust auto-ignition combustion. Generally speaking, for best fuel economy, an HCCI engine operates unthrottled and with a lean air-fuel mixture.
In an HCCI engine using an exhaust recompression valve strategy, combustion charge temperatures in each cylinder are controlled by trapping hot residual gas from a previous combustion cycle during a negative valve overlap (NVO) period. The NVO period is defined as a range, characterized by engine crank-angle, during which both intake and exhaust valves for a given cylinder are closed, and occurs around TDC-intake. Recompression during an NVO period occurs by advancing closing (i.e., earlier closing) of an exhaust valve, preferably in combination with retarding opening (i.e., delayed opening) of a corresponding intake valve, preferably symmetrical about top-dead-center (TDC), during each intake phase of a combustion cycle. Both the combustion charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous combustion cycle can be retained with earlier closing of the exhaust valve, which leaves less room for incoming mass of fresh air. The net results include higher temperature of the combustion charge and lower oxygen concentration of the combustion charge. The controlled use of NVO results in an ability to control the amount of hot residual gas trapped in each cylinder. During each NVO period, an amount of fuel can be injected and reformed in the combustion chamber.
In an HCCI engine with multiple cylinders, combustion phasing between individual cylinders can vary significantly due to differences in thermal boundary conditions of the individual cylinders, and differences in intake conditions, including variations in air intake, fuel injection, recirculated exhaust gases, and spark.
It is known to control combustion phasing by using extra heat released from the fuel reforming process to vary the cylinder charge temperature and the combustion phasing. However, excessive fuel reforming increases fuel consumption, and thus, it is beneficial to design a control scheme that achieves balance between cylinders with minimum combustion phasing errors using a least amount of fuel reforming.
There is a need for a system which improves performance of an HCCI engine while addressing the concerns described above.